Resolution
Resolution is made up of three things; dpi, pixels and inches.
Pixels = These are the exact number of "dots" vertically and horizontally that make up a photo. If you take a photo and zoom in very far, you'll see a bunch of squares. These are the pixels. The more pixels and dpi you have, the better looking your photo will be.
Here's another example. Say you took a small 2 x 2 inch piece of paper and started making dots in it with your pencil. If you make 10 dots it would just look like a piece of paper with some dots on it. If you made 10000 dots you probably wouldn't be able to see any more white on your paper, it would all be black. With photos, the more "dots" that are on your image, the more detail you will have. If you have few "dots", or pixels, your photo will look grainy.
DPI = The number of "dots" or pixels for each inch of the scanned photo.
Inches = The size of the scanned or printed photo.
So if you have a image that is 500 pixels wide by 500 pixels high and has a dpi of 100, the image will be 5x5 inches when it is printed.
It may be a little easier to understand by looking at this Image Size Menu from Photoshop.
At the top it says the image is 3600 pixels by 2400 pixels. At the bottom it says the Resolution is 300 pixels/inch. If we did the math, we could divide 3600 and 2400 by 300 and end up with an image that is 12 x 8 inches.
Another Explanation
Here's an example. In the photo below you'll see a picture that looks like a bunch of squares. These squares are the pixels. There are 4 pixels high and 5 pixels wide. If you look at the Image Size menu on the right you'll see it shows us that at the top. If you were to print this out, it would be a 5 inches wide and 4 inches high. The resolution at the bottom says 1. This means there is 1 pixel for every inch. In other words, if you were to print this photo, each square (pixel) that you see would be 1 inch wide and 1 inch high. Not a lot of detail here.
(click on photo to enlarge)
Now below we have a picture that is 25 pixels wide and 19 pixels high as seen in the image size menu. If you were to count each of the squares you would find this to be true. This photo has a dpi of 5 pixels for every inch, so if printed this photo would be 5 inches wide by 3.8 inches high. Since there are more pixels for every inch, this photo has more detail than the last.
(click on photo to enlarge)
Below is a photo with 10 pixels for every inch.
(click on photo to enlarge)
The photo below has 100 dot per inch (dpi) and the detail is very clear.
(click on photo to enlarge)
One more thing to remember. Your computer screen is only capable of showing 72 to 96 dpi. Any monitor, even the most expensive, high-end models, can’t show more than that due to the mechanics of the monitor itself. Therefore, if you have a photo that is 72 dpi and is 1600 by 1200 pixels, it may look great on your monitor, but when you print it out at 300 dpi it'll be about a 5x4 inches.
At What DPI Should I Scan My Photos?
So if you are wondering what dpi to scan your photos at, here is what I do. I like to have a printed image be at 300 dpi. This gives it enough detail to look nice and is a standard. Since I only like to scan my photos once (do it once, do it right) I figure I probably wouldn't want to print the photos any bigger than an 8x10 (this is the max). I probably wouldn't need anything more than that, so I scan my photos so that they are at least an 8x10 at 300 dpi. You may think that you will never want to print your photos that big and for most of them you probably never will. I, however, always like to error on too big of resolution than too small. Do it once, do it right. You may never have the opportunity to rescan your images. You're looking to preserve your photographs and if you approach it with that view and think "How would I feel if all of my photos were destroyed and my digital copy was the only one in existence?" you may decided to error on too big of resolution as well. You may think, you don't want to store a file that big because it will take up too much space. An 8x10 tiff takes up about 10 MB but technology these days is getting so advanced that you can purchase a 1 TB hard drive for about $130. 1 TB is 1000 GB. 1 GB is 1000 MB. So in other words, on a 1 TB hard drive you could fit 100,000 8x10" tiff photos. An 8x10 Jpeg takes up about 1 MB so you could fit 1,000,000 jpegs on your TB hard drive. I'll talk about this more later, but tiffs retain quality better so I personally like to save my photos as tiffs. For me at least quality is everything and I'm willing to take up a little more space for it, especially if it means I only have to do something once.
Now to find out how many pixels an 8x10 at 300 dpi is, simply multiply 8 by 300 and 10 by 300 and you will get 2400 x 3000 pixels. This is how many pixels you want to end up with.
Now if you are scanning a 4 x 6 print, and you want to end up with at least 2400 x 3000 pixels, divide 2400 by 4 and 3000 by 6 and that gives you 600 and 500. This tells you that you want to scan at 600 (the higher number of the 2). To find out exactly how big your 4x6" photo will be with a resolution of 300 dpi if you scan at 600 dpi, do the math. 4 x 600 and 6 x 600 gives you 2400 pixels by 3600 pixels and if you divide these by 300 dpi which is the standard that you want to print, you'll have an 8x12 inch photo. You may find that if you scan a print larger than twice it's size, it may not look that good. Test it out, but you may want to stick with only scanning prints double their size, so a 4x6 you would only want to scan to be an 8x12, 3x2 to be a 4x6 etc. In other words, you may want to scan all prints at 600 dpi max.
Let me give one more example but this time scanning 35mm film. 35mm film is aprox. 1.25" by 1". If you scan your 35mm negative at 2400 dpi (multiply 2400 by 1.25 and 2400 by 1) you will have an image that is 3000 by 2400 pixels. If this photo is then printed at 300 dpi, you will have an (3000 divided by 300 and 2400 divided by 300) 8x10 inch photo. Scan 35mm negatives and slides at 2400 dpi.
Megapixels and Resolution
Buying a camera and wondering how many megapixels you should get? I found this great megapixel calculator online http://web.forret.com/tools/megapixel.asp?
Here's what it tells me:
If you want a 2400 x 3000 pixel photo (8x10" at 300 dpi) you need 7.2 megapixels. If you want a 1500 x 2100 pixel photo (5x7" at 300 dpi) you need 3.2 megapixels.
When it comes to quality, error on the side of too big rather than too small. You'll be glad you did.
